Add behavior for encoding missing fields in Struct

[nozols]

Introduce an option to control the encoding of missing fields in the Struct constructor. This change allows users to skip encoding of undefined fields based on the specified behavior. Additional tests ensure the new functionality works as intended.

Note that fromBuffer doesn't support this, because we have no mechanism to determine if a struct field is omitted from the raw data.

[Copilot]

Pull request overview

Adds an option to Struct() intended to control how missing fields are encoded, and introduces tests for the new behavior.

Changes:

• Extend Struct(name, defs) to accept an optional opts argument with encodeMissingFieldsBehavior.
• Update Struct#toBuffer() to conditionally skip encoding undefined fields when configured.
• Add unit tests covering default behavior and the new skip mode.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

File	Description
lib/Struct.js	Adds opts parameter and a conditional skip in the encoding loop.
test/Struct.test.js	Adds tests for default encoding of missing fields and skip behavior.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

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